Can you give me a bit more "walk through" of what is prompting for this value and the context? I think you get that from your Webmasters Tools management area, but I want to make sure we are talking about the same thing.
It looks like you login to the Webmaster tools and then click on your domain name listed below. If you haven't added a site, you need to add that site first. When you click on the domain, it gives you a code like the one listed below.